B (3:42)

Yeah, absolutely. I read in another interview that you started with, obviously a first draft, and as you refined it, it got more, I guess, accessible. And I do think you do a really nice job of balancing the amount of information without overload. It's certainly not a textbook. I want to ask the obvious question. What is the tree of life?

C (4:05)

So the tree of life is a fairly easy concept to understand because we're all familiar with genealogies. So family trees, how are you related to your brothers and sisters and your cousins, your parents, your grandparents? So we can think of that as a tree like structure. You've got your great grandparents and they had various children, one of which is your grandfather or your grandmother. They had various children. So you're starting off with somebody in the past and that branches out to his or her children, grandchildren, and getting bigger and bigger. But the tree of life is just that, on a grander scale. So we can go back to the very beginning of life on Earth and think about the original species. And I think we might talk about that later. There's an organism called luca, the last universal common ancestor. That's as far back as we can go. But LUCA at some point was a single species, a population of probably billions of individuals. But that population at some point split, and you can think of that as the trunk of a tree dividing into two branches. And those branches would go on to found great domains of life which would give rise to very different species much later. And so essentially we can Think of the Tree of Life growing upwards, dividing into making more species, and those species becoming different from each other. And the Tree of Life describes that process. It describes how species divide to make new species and how those species are related to each other.

B (5:42)

We absolutely will talk about luca. Before we move on, I do want to ask, do you consider this a book about the story of evolution? Here I want to quote a sentence you have. In one of the early chapters, we will witness three important aspect of the growth of the Tree of Life, how new species are created, the evolution in these new species of new characteristics, and the passing down of these novel characteristics to future generations.

C (6:07)

Yes. So there's two things there. There's the process, which is, in the sense what you just described is in a general process, Darwinian evolution, where a species, some anonymous species, devised to make two new ones. Those two new ones are the foundational members of new branches of the Tree of Life and they will become different from each other. Once they've separated, they'll invent new characteristics in the Darwinian way. So they might invent, one group might invent a backbone and another group might invent wings or something. It might become an insect and one might become a vertebrate. And those characteristics then get handed on to their descendants effectively and define the branches that they come from. So the backbone, once invented, was inherited by all of the vertebrates. And so it kind of tells us, which is a big clue as to, if you find an animal, is it a vertebrate or not? If it has a backbone, then it is. So there's a sort of a general process going on there which happens right across the Tree of Life. And it's a mechanism rather than what actually happened. But the second part of the story is what actually happened. So how can we reconstruct the events, the specific speciation events, the specific invention of new characters, the specific formation of new branches, and obviously, you know, there's millions, billions, possibly trillions of species. So I didn't want to set out to describe the whole of the history of life. What I really wanted to do is to show how we can study that history of life.

B (7:46)

Why is the tree diagram the ideal form of this representation?

C (7:51)

I don't know. Just seems to work extremely well because it's very good at describing those processes. It's very good. A branch splitting is a perfect way of explaining speciation or depicting speciation and those branches growing upwards. The length of those branches as they sort of reach up towards the sky is how we depict change in evolution. So Those facets of a tree are exactly what we want to explain in terms of the evolutionary process. And so the tree does a brilliant job of that.

B (8:27)

And I believe Darwin may have sketched or referenced a tree, but do we attribute the first actual diagram to Ernst Haeckel? I hope I have that right.

C (8:39)

Well, so Haeckel, who was known as the German Darwin, who absolutely adored Darwin, I guess, arguably drew the first evolutionary tree. So we had trees showing similarities between organisms before Darwin, even evolutionary trees without the Darwinian mechanism, Lamarckian trees, for example. But post Darwin, Haeckel was the first to draw what we can consider an evolutionary tree relating species.

B (9:11)

I want to read just a great line. Always love the humor in any book. Like Darwin, Heckel, at the insistence of his father, trained initially as a medical doctor, but abandoned medicine soon after meeting his first patients. Perhaps the research for his doctorate, medicine, which is awarded for a study of crayfish, left him ill prepared for the realities of the human body. Rather like the story of John Ruskin on his wedding night. So that was. That was a very nice side. Can you describe our model of speciation?

C (9:44)

Yeah. So in the sense if you've got a single species and you want to divide it into two, you've got a problem because all the members of that species are reproducing with each other and mixing their genes up. So this is continually renewing itself as a single population with interchanging bits of DNA. And so what we need to do is to divide that in two. We need to hive off a bit of that population. It could be half or it could be a small bit and stop it reproducing with the other bit of the population. And as soon as that happens, those two separate things which are no longer mixing their genes up through reproduction, are able to start to become different from each other. And that can happen in many ways, but the sort of. The easiest way to think about it is if some sort of barrier comes up. So you could imagine a big population of birds all happily mating with each other and muddling their genes up. But if a huge mountain range appears in the middle of them, that divides the population in two. One population on the west side of the mountains and the population on the east side of the mountains are no longer going to be able to mix their genes up. And from that point on, they're able to start to evolve through the Darwinian process to be different from each other. And so we think of this as a sort of classical way of one species dividing into two, essentially a population of One species being separated so that they can no longer mix their genes up.

B (11:17)

You talk a fair amount about the process for building out the tree of life. Could you describe the role of fossils in building out the tree of life?

C (11:25)

Well, fossils, they're just, in many senses, they're just like the things that are alive around us. So we look at the things around us today and look at the characteristics they have and use those characteristics to decide which groups they belong to. So I've talked about the backbone as a characteristic that tells us that all these animals over here that have a backbone are related to each other in a group called the vertebrates. But we can look at exactly those characters in fossils. We can find fossil fishes that have a perfectly good backbone, and we can see that they must therefore have belonged to the vertebrates, although we can't actually see we can't meet the animal that produced the fossil today. They've gone extinct a long time ago. But we can still classify them in exactly the same way that we classify everything that's alive today. And we can do that even for groups like the trilobites, big group of arthropods, so things with jointed legs related to the insects and crustaceans. There are no trilobites around at all today, but they were hugely diverse in the past. And we can use exactly the same principles. We can look at the characteristics different groups of trilobites share and use those characteristics to work out the relationships between the different groups of trilobites and use those, use those relationships to reconstruct a family tree of the trilobites. So we've got this branch of the arthropods, effectively all of which are now extinct, but which we're perfectly able to add to our great tree of life as an offshoot of the arthropod branch.

B (13:04)

I was surprised to realize DNA really isn't as helpful as the movies or Hollywood might suggest it is here. I'm thinking of Jurassic park, specifically. What is the problem with DNA and sort of working backwards to build out the tree of life?

C (13:20)

Well, we use DNA a lot today because it's a very good way of working out what's related to what. So, I mean, in a very simplistic way, you can say, how similar is my DNA to that of various different animals? And my DNA is very similar to a chimpanzee, not quite so similar to a gibbon, less similar still to an earthworm or an insect, et cetera, et cetera. So DNA contains a massive amount of information because there's so many little subunits to the DNA. There's 3 billion letters in the human genome. So an awful lot of information. That's all very well for things that we can go and grind up today and extract their DNA and compare it to other living creatures. But DNA doesn't really fossilize. So Jurassic park was using the DNA sucked out of a dinosaur by a mosquito 65 million years ago. That's all more because that's when the dinosaurs went extinct. But DNA literally just doesn't last that long. It lasts a very long time, tens of thousands of years, but it's not millions and it's certainly not billions of. So we can't use DNA to look at the trilobites as we were just talking about, because they went extinct far too long ago and all their DNA will have degraded a very, very long time since.

B (14:45)

Can you discuss the significance of the discovery of Latimeria chaumnae?

C (14:51)

Sure. So Latimeria is. Well, for a long time it would have been thought of as a fossil. And that's the interesting thing about it. So Latimeria was discovered by Marjorie Courtney Latimer in South Africa. She ran a museum and she cooperated with a local fisherman called Captain Gooseens, who would drag up fish in his nets and she'd sort through them. And if there were any interesting samples there, she'd stuff them and put them in her museum. And one year, I can't remember exactly when, early 20th century, just before Christmas, he rang her up and said, I've got an interesting looking fish, see if you want to come and have a look at it. She'd never seen one like it before. It was quite a large One, about 5ft long, silvery with white spots. And she recognized immediately that a, that she hadn't seen anything like it. They didn't have anything like it in the museum. But it did remind her of some fossils she'd seen. And this is what it turned out to be. It turned out to be what we now refer to as a living fossil. And essentially it was almost identical to fossils from 400 million years earlier. These are the sarcopterygians, which essentially, sarcopterygium means fleshy finned. And see, these are fish that have quite stout fins on them, which seem to have been the structures that eventually would evolve into our own arms and legs. So Latimeria essentially was a fish that had survived almost unchanged for hundreds of millions of years. She got it to an ichthyologist friend of hers who said, yes, this is an amazing fossil. I can't believe these are still existing.

B (16:48)

Excellent. I wanted to ask you about the term homologues and why they're so important in evolutionary biology.

C (16:56)

So homologues are really interesting. They predate evolutionary ideas. So Aristotle recognized characteristics of different species of animal that looked rather similar. So an easy example is the wing of a duck and the wing of a swan. They're rather similar. And so it doesn't take much imagination to imagine that these are kind of different versions of the same thing. And so this is what homolog referred to for a long time. But once we had Darwinian evolution and we realized that species were related and the characters got handed down from ancestors to descendants, homologues took on a different flavor, if you want. So what homologues are a character that existed in an ancestor and now have been passed down the tree of life and now exist in the descendants of that ancestor. And so I keep talking about the backbone. The backbone evolved once, maybe 500 million years ago in the ancestor of all the vertebrates, and it has been passed down with different variations to all the vertebrates since. And it's these characters, these shared characters that have been inherited from a common ancestor that actually tell us the shape of the tree of life. So it's these characters, these homologous characters, that tell us how things are related. There's a sort of interesting distinction here because there's another sort of character that looks similar in different organisms. And these are analogues, these are analogous characters. These are characters which weren't inherited from a common ancestor, but look the same because two branches of the tree of life have evolved the same similar looking structure. So a trivial example would be the wing of a bird and the wing of an insect. So these are kind of the same thing. They're flapping structures that enable an insect or a bird to fly, but they're clearly not the same thing. An insect wing doesn't have bones or feathers. And so that's a sort of trivial example of an analog, which, if we were very silly, might make us imagine that bees and birds were closely related. And of course that's a silly example, but there's lots of very good examples where things do look very similar. And it's very hard to untangle the fact that they're similar due to convergent evolution rather than. So they've convergently evolved a similar looking structure rather than being closely related. And having inherited that similar looking structure from a common ancestor.

B (19:40)

You write, the way of thinking about homologues uses a simple but important, logical trick called the parsimony principle. Could you explain that?

C (19:47)

Yeah. So you can think about three species, a bird, a human, and a goldfish. And you can think about a character that are shared between some of them. So birds and humans have legs and fish don't. And so we can use those characters, that character, to try and work out the relationships between a bird, a human and a fish. And obviously, we all know the answer to this. You know, birds and humans are things with legs, and they're close, more closely related to one another than either is to a fish. But this is, we can sort of the theoretical underpinning of that is that trying to imagine how legs evolved in humans and birds and not in fish, if a fish was related to a bird, rather than being the sort of more distant ancestor of those. And so if a bird and a human are close together, then we have to think of legs evolving one time. But if birds are more closely related to fish, then we'd have to think that legs evolved twice, once in birds and once in humans. And that's less parsimonious. And that's what Occam's razor is. Essentially. It's trying to pick the model of how evolution happened by choosing the simplest model.

B (21:10)

You write. And I'm shifting to discussion on genes and the DNA for building out the tree of life. Biochemical characteristics have been a useful complement to morphological characters for reconstructing the tree of life, but are, if anything, even fewer in number or more hard won. Almost any biochemical component you might want to study requires its own specialized equipment. What we really want is a source of characters that are quick and easy to read from any given species. With that in mind, what are the benefits of using molecular characters over morphological characters to build out the tree of life?

C (21:46)

The biggest advantage of using molecular characters, which are the DNA molecules and the protein molecules. So the sequence of four letters in DNA, A, C, G and T, and the 20 letters arranged along in a linear sequence in proteins is that there are just millions of them. So if I try and compare a bird and a bee, I might be able to find a couple of dozen characters which I can compare between the two of them. But if I wanted to look at the DNA of a bird and a bee, I'd find hundreds of thousands or millions of letters which I could compare across their DNA, across their proteins. So that's the sort of biggest advantage of using molecular characters. That's just a huge amount of data. The second thing, I've talked before about convergent evolution and you can imagine evolution working on making a wing, and there's various ways of doing that. And wings can end up looking very similar, like a bird and a bat wing. So that's a terrible, terrible example.

B (22:56)

Max, we've just discussed part one. What is the Tree of Life? I want to move to part two of your book, how to Travel in Time. And there we get to what you referenced there earlier is this notion of the last universal common ancestor, otherwise known as luca. Could you talk about what that is and how we determine it? I really found that one of the most fascinating parts of the book.

C (23:19)

So what I mean by traveling back in time is essentially what we can do is by looking at how organisms are related and what characters they share, we can work out if we go back to the common ancestor of any pair of species, we can look at what characters they share and say, well, those characters must have been inherited from that common ancestor. So if I look at a human and a chimpanzee, we find a lot of characters shared because we're very closely related. And we can look back in time towards the common ancestor of a human and a chimp and say, well, we must have been apes without a tail and, you know, various other characteristics like that. And then we can do that over and over again. So I can go and say, well, what characters do I share way back in time with a jellyfish? This is a much more distant animal relative of a human. But nevertheless, we share things in common with jellyfish. We have a gut, we have muscles, we have nerve cells, we have multiple cells, we have lots and lots of cells. We have large bodies. And so we can go, we've just traveled back in time maybe 600 million years to the common ancestor that we share with jellyfish. And we can keep doing that back and back and back and back. And eventually we get, for any pair of species, we get to the most distantly related possible ancestor that those have. And at some point we can't go back any further. This is the earliest split on the tree, and it actually comes between two groups of rather similar looking organisms. In a way. These are the eubacteria and the archaea, which used to be called the archaebacteria. And those are the most distantly related branches on the tree. That's the very earliest branch. And so we can travel back, we can look at those two most distantly related branches, say what's in common between those, and extrapolate those similarities right back to this first branch in the tree of life. But we can't go back any further because we can't make any more comparisons. And that furthest back point we can make is known as the last universal common ancestor. So it's an ancestor from which everything alive today originally emerged. And it's very interesting. Okay, so it's a long, long way back in time. We're comparing very, very distantly related organisms. These are organisms, two branches of the tree of life, which split apart 4 billion years ago. But nevertheless, we can find some really important bits of biology that are shared across those two branches and in fact, are shared right across everything alive today. So I've talked a lot about DNA. So the double helix is common to all of life. Must have been there in luca. In other words, the way that DNA gets turned into proteins, the way genes work is identical across the tree of life, so must stem from LUCA as well. It's common to the whole of life. And there's various different bits, bits of biochemistry, bits of cellular machinery, which are really, really ancient. And we can work out that they must have existed right back 4 billion years ago. And in fact, these fundamental, complicated similarities are what tells us that there is one tree of life, that there is one origin of life, and that ancestor gave rise to everything alive today.

B (26:50)

Okay, Max, I want to ask you about ssurna and how did it factor into Carl Wosay's discovery? And he had an assist from Margaret Dayhoff, because this gets into that branching. We, I think, originally conceptually thought it was a one split leading to two branches, but his research suggested it actually was three. Could you talk about that?

C (27:15)

Yeah. So Carl Ways is an American scientist working in the 1970s and on. But his big discovery, or his most famous discovery, is from the 70s. And what he was doing was comparing what we really thought of as the two different kinds of life. There's something called prokaryotes, which are basically the bacteria, very simple, very tiny. They don't have a cell nucleus. Prokaryote means before the nucleus. So that's one branch and the other branch of the eukaryote. So these are more complicated cells. They have a nucleus. They have a little cellular structure called a mitochondrion, which is where the energy comes from. And we are eukaryotes. So we are one very complex, very large example of a eukaryote that has many, many cells stuck together. But if you look at our individual cells, if you look at one human cell, you'll find it's got the same bits that all the eukaryotes have. So it's got A cell nucleus with chromosomes in it's got these mitochondria for making energy and various other complications. So that's where Woze was starting from, that the idea that there were two kinds of life, prokaryote and eukaryote. And what Woese did was essentially go in and look at a gene called small subunit ribosomal rna, which is a structural. It doesn't actually cave for a protein like most genes. It's actually a structural thing that helps make another cellular structure called the ribosome. And he went in and very, very painstakingly. It was very difficult at the time. He characterized the small subunit ribosomal RNA of lots of different bacteria and lots of different eukaryotes. And what he found is essentially all the bacteria had rather similar SSR small subunit ribosomal RNAs. They were similar to one another, and they were very different from the eukaryote ribosomal rna, which kind of makes sense. So this sort of supports this split in two. The weird thing came when he studied these very odd kind of prokaryotes. So these bacteria kind of. Of life called the methanogens. And the methanogens he found when he discovered their. When he studied their ribosomal RNAs, all looked fairly similar to each other, which made sense. They were obviously relatives of one another, but they were as different from the eukaryotes and the bacteria proper as those two were from each other. So essentially what he discovered was not two branches of life, prokaryote and eukaryote, but three branches of life, prokaryote, the bacteria, eukaryote, the complicated cells, and a third domain of life called the archaea, the ancient ones. And this is a total surprise. And many people thought he should have won a Nobel Prize for this discovery.

B (30:23)

Yeah, I think he may have been one of those people. But that's a great story. Max, I want to move on to a question you pretty much title the chapter. One of your chapters, namely how insects got their Wings. Another really great section. Could you briefly talk about that process? I. I think, you know, it might. It might not seem as straightforward as we think it is.

C (30:46)

Yeah. So insects are. Almost all. Insects have wings. Some of them have lost them. Some stick insects have lost their wings. Wings are. Insect wings are actually, you know, we think of them as things for flying, but they use them for all sorts of things. They use them for signaling and camouflage and many other ways. But we've always been interested in where they came from. And there were essentially two big ideas. The first was that it was just the top of the carapace of an insect was slightly flared, slightly stuck out to the side. And this enabled this early insect to glide a little bit. And then as you can imagine that, that getting bigger and bigger the gliding got, getting better, the flying getting better. And eventually the insect would attach muscles to that flared bit of the carapace and start flapping them. So that was one idea that it came from an expansion of the carapace. The other idea was that it could, the insect wings could have evolved from a structure that we see on the base of the legs of their relatives, the crustaceans. And these are gill. So gills are a little bit similar to wings in that they're kind of flat structures. They even get wafted around by an aquatic crustacean to get oxygen out of the water. They're also positioned in roughly the same place that wings sort of grow from on an insect, near the bottom of the legs. So in a crustacean, the gills are attached to the bottom of the leg and in an insect that they sort of moved a bit, but that's where they originate when an insect is developing. And recently it's been possible to test those two different hypotheses. And what we found is that the genes that are involved in an insect making an insect's wings are very much overlapped with the set of genes that are involved in making a crustacean scills. So it really does look like the second idea is correct, that there was a, a crustacean, maybe a crustacean that was coming to live on land a little bit and it had these tiny gills and those gills were what actually became adopted by the insect to change their form, change their function, and turn those pre existing structures into wings.

B (33:10)

Yeah, that was a fascinating read. Max. Why can't a member of one species reproduce with another species member? And even if they could, why would this be a mistake or a disaster?

C (33:23)

Well, the reason they can't in general is because of this process of speciation. So one species splits into two. Those two species start to become different from each other. And the longer they've been separated, the more different they are. And you can imagine a male of one of these different species and a female of the other coming together to try to reproduce. And they're mixing up their genes, but their genes, especially if they've been separated quite a long time, have become adapted to work within the genome of species A or within the genome of species B. And when you try and mix them together, the components don't mesh together. And I'VE sort of liken it to trying to make a watch out of half of a Rolex and half of a Timex. And that watch is not going to work very well because all the Rolex parts go perfectly well together to make a Rolex and all the Timex parts go perfectly well together to make a Timex. But try and mix them up and you get a disaster. That doesn't mean it doesn't happen. So there are some really important bits of evolution in the early evolution of the vertebrates. For example, we can see that the number, it's happened in a different way, but the number of genes has actually doubled at some point. And that happened by all the genes with one species A and all of the genes with one species B getting put together in some sort of weird hybrid. And that injection of new genes is thought to be a big reason for the sudden evolution of the vertebrates and the fact that vertebrates are quite complicated animals.

B (35:10)

Yeah, I want to ask you about that because there, at some point, you suggest the process may have accelerated, but use a nice phrase, Franken Watch, to describe the, I guess that hybrid, potential hybrid structure. We've talked about a few individuals, there are some colorful ones, and candidly, I guess there's some less colorful ones or nasty ones, but they also played a role here. And I want to talk about Konstantin Mariszkowski, putting aside his own background. Could you talk about his insight into the origin of eukaryotes and then the work by, I think, Carl Sagan's wife, Lynne Margolis, on furthering that thesis?

C (35:51)

Yeah. So Konstantin Mere Chovsky was a horrible person. I don't really want to go into the details of the horrors of his life, but he was a pretty evil man. But he did come up with some very interesting and important ideas in evolution. And that idea is called endosymbiosis. So endosymbiosis means endo means inside, and symbiosis means living together. And so essentially, his idea was an explanation for where the chloroplasts in plant cells come from. So all plant cells, or certainly all the. All the green plant cells, have little structures inside them shaped like a lentil called a chloroplast. And these are green, and this is where photosynthesis happens. And so Maroszowski's great idea was trying to explain where these came from. And what he noticed was that the chloroplasts looked very similar to a weird kind of bacterium called cyanobacterium. And these are the bacteria that are able to photosynthesize, so to suck in carbon dioxide and use sunlight to convert that into carbohydrates and oxygen. So a very important process in life as where all our food comes from, where all our oxygen comes from. And essentially what Marechovsky suggested was that a eukaryotic cell that wasn't able to photosynthesize at some point, engulfed, swallowed one of these cyanobacterium. And the cyanobacterium lived on, didn't get digested, but lived on happily inside this eukaryotic cell. And this was the founder of all the photosynthesizing eukaryotes, so algae and dinoflagellates and all the plants, et cetera, et cetera. And so this idea that an organelle, a little part of a plant cell, could have arisen by one species engulfing another species, and then those two living together forever after was a very exciting idea. And this idea was taken up in the 60s by Lynn Margulis, who was briefly married to the astronomer Carl Sagan. That was very famous in her own right, of course, and she took this idea on and expanded it. I've talked a little bit about these structures in eukaryotic cells called mitochondria. They look like little subaqua cylinders or something like that, or submarine, people sometimes describe them. And these are where all our energy is produced. So the mitochondria absorb carbon dioxide and burn it in oxygen to produce. Sorry, they absorb carbohydrate, burn it in oxygen to produce energy with carbon dioxide as the waste product. And what Lynn Margula suggested was that mitochondria, this essential, hugely important bit of a eukaryotic cell, had evolved in much the same way as the chloroplasts in a plant. Essentially, a eukaryote had swallowed a bacterium, a different sort of bacterium called the Alpha proteobacteria, and that bacterium now lives inside our own cells. So the mitochondria that all our cells have are actually a different species from us. So this is a species of bacteria living permanently inside all our cells and producing all the energy that we require to live.

B (39:37)

I want to move on to when trees go wrong. And here you discuss the issue of long branch problem. Could you talk about that?

C (39:46)

Well, I think to step back first, we need to think about something I talked about earlier, which is analogy. So this is the same character evolving in completely unrelated species. And the example I give in the book is looking at swallows and swifts, so two birds, which I'm not an ornithologist, two birds, which to me look very, very similar. And I assumed always that they were very closely related, but it turns out that they are very distantly related. So a swift is more closely related to a hummingbird and a swallow is more closely related to an owl than they are to each other. So they're very different, they're very distantly related, but they have come to look very, very similar because they live very similar lifestyles. And so this is essentially the problem when we've got a single character evolving in two unrelated branches. And this can happen all the time. What we have to do is collect lots and lots of data and hopefully we'll get enough characters that are telling us the real relationships and these will sort of swamp out the characters which are misleading us. And so this is the source of all the errors on the tree and what we have to deal with. Now, long branch attraction is a specific case where we get a lot of this convergent evolution happening on branches of a tree. And it's quite a complicated topic. But the basic problem, the basic thing that happens, that misleads us is that very fast evolving species end up looking very different from their close relatives. But occasionally, because they're fast evolving, because there are lots of things that are happening in their genomes or on their morphology, they end up looking similar to one another. So these are sort of random instances of convergence which make them look more similar than they really are. And what essentially happens is that when we look at, when we try and reconstruct the tree and work out what's related to what, we accidentally put the fast evolving species together, even though they're not closely related.

B (41:58)

In this discussion of the long branch attraction problem, you talk about convergent evolution. You referenced it earlier in this conversation. On one hand you describe it as the grit that makes the pearl, but you also say it's a double edged sword. Why is that?

C (42:14)

Well, it's a double edged sword, okay, because it has both good things and bad things about it. So the bad things are what I've just described. It makes us get the tree roll. So if we've got convergent evolution, the same character evolving in two unrelated species, then it fools us into thinking they're closely related. And that's bad. But the good thing about it, the good thing about convergent evolution is it gives us separate examples of the same thing happening over and over again. And so this can start giving us clues as to why things happened. So most of the time we can see that such and such a thing evolved and we can have a pretty good guess as to why it evolved. But in order to test that, to test our ideas of the reasons behind different characters evolving, we need to have, ideally, lots of examples of it. So if we can say big ears evolved in this fox because they need good hearing because they're predators, that's very plausible. But if we always find big ears in all sorts of different predators, in cats and weasels and all sorts of other things, then that's sort of confirming our hypothesis, because there could be other reasons for the big ears evolving. But if big ears always co occur with a second characteristic, listening out for prey, for example, then that convergent evolution of big ears, that fact that it's happened over and over again and it always coincides with being a predator, then that kind of confirms our view of why it evolved.

B (43:58)

I want to move on to discussion of determining the rate of change. To measure the passage of time, you suggest we could use both morphological or genetic change. Why is genetic change a better measure to measure the rate of change?

C (44:16)

So if we want to use how fast things are changing to measure time, well, there's a fairly obvious correlation between lots of time passing and things becoming more different. So we could use how different we are. If it was evolving in a very regular fashion, we could use how different we've become from various ancestors to work out how closely related we were. And the problem comes from the fact that change isn't very regular when we think about morphology, and it's kind of implicit in what we've been talking about. So we talked about Latimeria chalumni, the coelacanth, Sorry, this worm, this fish that had barely changed in 400 million years. But in exactly that same 400 million years, the same ancestor that produced the coelacanth also produced humans. And so what you can see is that in one branch of the tree, almost no change had happened. 400 million years, no change, you've still got a coelacanth that looks exactly like a fish. And in the other branch, over those same 400 million years from that same ancestor, we have evolved into a human and a giraffe and a frog, for example. So what that tells us is that in some branches, morphology changes barely at all, and in other branches, morphology changes very quickly. And so essentially, morphology doesn't tick regularly like a clock. Some branches it ticks very quickly, some branches it ticks very slowly. And so it's not going to be a very good way of measuring time. But if we look at molecules, what we can see is that the changes to genes happen much, much more regularly. So there's two advantages of genes. One is that they tick the changes that happen to genes. The tick is much more regular, like a clock. And the second thing, of course, is what we've discussed before is that there's an awful lot of characters in the DNA that are all ticking away, giving us separate measures, independent measures, of how much time has passed. And so essentially this allows us to travel back in time and know at what time a common ancestor existed. So when did the first animal appear? When did the first vertebrate appear? When did the first mammal appear? What was going on on Earth at that time? Once we know when it happened, we can start trying to correlate it with other events on the tree of life or events on the Earth.

B (46:45)

For example, how closely are humans related to a little weird worm with no brain called xenocoelomorphic? But.

C (46:53)

Well, this is one of my favorite groups of animals because it's a very obscure group of worms, very, very simple. And there's sort of been two, two views about where they belong, this very, very simple group of worms. Most people, I think, probably I'm in a minority here, think that they're one of the earliest branches of the animal. So they're very simple because they evolve very early on and haven't changed since. So they missed out, they branched away before the invention of lots of complex bits of anatomy that most other animals share. The other view, it's my view is that these very simple worms, there's a worm called Xenotabella and its various relatives, these very simple worms, are actually relatively closely related to the chordates. So this is the group to which we belong. And chordates have a head and a tail and a gut and all sorts of things the Xenotabella and its relatives don't have. And so my view is that this is an example where an animal has lost characteristics, has become less complex. And this is another way of fooling us about where animals belong in the tree. If you throw away all your. All the characteristics you share with your nearest relatives, then you're going to appear to belong somewhere much lower down in the tree as a much more early branch, a simpler kind of animal. And this is what I think happened with Xenosabella. I think it evolved from a much more complex animal and has lost many characteristics. And this is what's fooling us to thinking it's a much earlier animal.

B (48:40)

Hmm, interesting. You start part three of the book titled Tracing Our Family Tree by noting evolution isn't purposeful, would you say? It's random.

C (48:51)

It's random in that there's no guiding what happens. There's no changing a bit of DNA in order to make a specific change. So the changes that happen to DNA that lead to changes in morphology are completely and utterly random. They're mutations, they happen by mistake. The process of evolution by natural selection that Darwin described makes that look very purposeful because only the good mutations, of all the random mutations, only the good ones survive. And so animals, species get better at dealing with their environment, make interesting new inventions like backbones and fur and teeth, et cetera. So it looks purposeful, but it's just a sort of emergent property of the process of evolution by natural selection. The other point I was kind of making was that if, as you do, if as I do at the end of the book, and trace the path up the tree from luca, the last universal common ancestor, all the way up to humans, you can imagine that we were deliberately adding first a nucleus and then having many cells, and then deliberately adding all these characters one after another in order eventually to arrive at humans. But the point is that that's completely random and it sort of looks more purposeful simply because I've picked a certain path through the tree in order to end up at a human, and I could have picked any root of the tree and ended up at any other species at random.

B (50:33)

You note the transition from water to land and air and the evolution of four legs is one of the most famous and deeply studied events in all of evolutionary biology. All land living vertebrates come from these Devonian ancestors. What is so significant about this transition from water to land and the development of four legs?

C (50:54)

Well, it's kind of significant from a human point of view because we're part of that transition. So we have four limbs. Our arms are no longer legs, but they're the same thing. So we belong to this great group of vertebrates called the tetrapods, which simply means four legs that evolved from a group of fishes. In fact, the group of fishes that we evolved from include the coelacanth Latimeria that I've talked about. So this is, as I've mentioned, part of the group called the sarcopterygia. These fleshy fins and these fleshy fins in Latimeria, in a different branch of the Sarcopterygia would slowly evolve into our arms and our legs. And actually, if you look at the bones at the base of the fin of Latimeria, we can see counterparts of our humerus and radius and ulna and our fingers. So this took a long time. There's a very interesting set of transitional fossils where we see things becoming much less fishy and much more like a proto frog or lizard, as our ancestors slowly, tentatively, spent more time out of the water on land, and their legs grew stronger and eventually ended up producing something called the pentadactyl limb, the five fingered leg, five fingered limb, which is essentially the pattern that all four legged animals alive today share. So we all have radius or a humerus at the top, or a femur if you're a back leg, a radius and ulna and then five fingers. And so with a few exceptions, where fingers have been lost, like in some birds, in the birds, we all have five fingers, we all have a radius and ulna, and we all have a humerus which we can recognize. And this is true from everything from the human arm to the flipper of a whale to the wing of a bat.

B (52:59)

Was losing our tail an evolutionary benefit or was it actually some kind of break or mistake?

C (53:06)

I don't think we can know, but we know genetically how it happened. There's a gene called brachyuri, which just means no tail, because that's the effect of mutating the gene. So at some point in our ancestry, in the ancestry of all of the apes, all of the primates that don't have a tail, so that's gibbons, orangutans, chimpanzees, gorillas and humans. So in the common ancestor of all the apes, these are the monkeys that don't have a tail. There's been a mutation happened to this gene, brachyuri, which is what caused the tail to fail to develop when we were embryos in the womb. What the benefit of that was is lost in time. Lots of ideas for why it might have actually been beneficial, but maybe it just wasn't detrimental. So maybe it happened. There was a mutant. That mutant, despite having no tail, thrived perfectly well. And that's what's been handed on to us ever since. So we can see examples of that. There's the Manx cat from the Isle of Man in the British Isles, which are cats without a tail. And they have a mutation in exactly the same gene, the brecchieri gene. The Manx cats seem to get on perfectly well without that tail.

B (54:30)

Very nice. And I'll give you a chance. You suggested if you could have a tail, is there a particular one you would prefer?

C (54:38)

Yeah, well, I quite like a big, bushy, stripy one like the lemurs. I think it'd be quite nice to have a tail sort of things, pick things up with it, you know, we probably have special hairdressers for our tails. Wouldn't we have tails?

B (54:53)

Yeah, I could see a whole industry developing. They make long distance travel difficult, but I'm sure we would adapt. Speaking of adaption, how much of Homo sapiens survival can be attributed to luck versus our perceived intelligence?

C (55:08)

Well, it's difficult to know, but there's evidence recently suggesting that we very, very nearly went extinct. So there's plenty of other species of Homo Homo neanderthalensis, Homo denisova, et cetera, which have gone extinct. And, you know, at some point we were just another one of those species, maybe slightly cleverer, maybe not, but there's evidence that, you know, points in the past, there were as few as a, you know, a couple of thousand Homo sapiens hanging on by the skin of our teeth for, you know, many thousands of years and anything could have happened to those, that tiny group of humans to make them go extinct. So it's rather lucky that, rather lucky for us that they managed to pull through and obviously have gone on to great things. There's now 8 billion of us, which is not necessarily great for the rest of the planet, but we're around today to look back on our history.

B (56:14)

Last question. You note we've only classified 1 million living species out of an estimated 9 million. How do we close that gap?

C (56:23)

Well, one thing we're doing is speeding things up by rather than carefully describing things and people being expert, we can just grab the DNA of things and classify them according to the DNA. So every individual species has its own unique DNA, which will be very similar to its closely related species, but will be unique to its, will have some unique characteristics. And so if we, we can go and collect any species, grind up its DNA and know it in that very simple way. This is rather a shame though, I think, because we're sort of losing the skills of the taxonomists. These are the people who really know these organisms, know not just the sequence of DNA, but know the morphology, the structure of their cells, the subtle differences between this species and that species. So automating it and just doing it with DNA is a very good way of cataloging everything, but it's not a very good way of knowing the diversity of life.

B (57:25)

Great, that concludes the interview. Max, this has been a really interesting conversation. The Tree of Life solving science greatest puzzle is a really thought provoking work and I think everyone can learn a lot about our background and our history. Thank you so much for joining me today to discuss your book.

C (57:43)

That's been a great pleasure. Thank you very much.

D (57:58)

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C (58:26)

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